An extrusion process is one of the most economic methods of manufacturing to produce engineering structural materials. Typically, an extrusion process is used to manufacture lengths of extruded members having a uniform cross-section. The cross-section of the members may be of various simple shapes such as circular, annular, or rectangular. The cross-section of the members may also be very complex, including internal support structures and/or having an irregular periphery.
Typically, an extrusion process utilizes thermoplastic polymer compounds that are introduced into a feed hopper. Thermoplastic polymer compounds can be in powder, liquid, cubed, pelletized and/or any other extrudable form. The thermoplastic polymer can be virgin, recycled, or a mixture of both. Furthermore, the thermoplastic material can be incorporated with a blowing agent(s) or mechanically injected gas during the extrusion process to make a cellular foam structure core.
A preferred material used to form the core is a rigid PVC powder compound that is easy to process, good impact strength, a high extrusion rate, good surface properties, good dimensional stability, and indentation resistance.
In addition, a preferred extrusion formulation may contain one or more processing aids. One example of a preferred processing aid is an acrylic based resin having a low molecular weight, such as Acryloid K-125 or K-175 from Rohm and Haas. Also, one or more lubricants may be used. An internal lubricant and an external lubricant may be provided. Preferred internal lubricants include metallic stearates such as calcium and zinc salts of stearic acid. Preferred external lubricants include paraffins.
Additionally, fillers may be added to the thermoplastic formulation to reduce product cost and to improve impact properties. Although many types of filler are compatible with the thermoplastic resin, a typical filler is calcium carbonate.
Examples of uses for extruded members include extruded composite building materials. Extruded composite building materials have been used in applications of house siding, architectural moldings, fencing, decking, and other applications. One drawback associated with existing extruded composite structural building materials is that existing materials lack the strength necessary to directly compete with or replace structural wood, e.g., various sized wood beams, i.e., 2×4, 2×6, 2×8, 4×4, 4×6, 4×8, etc. The environmental stability of composite materials, i.e., no dry rot, no termite, no warping, no splitting, etc., has resulted in increased popularity of composite decking and fencing materials. However, composite materials typically still require wood support structures for structural strength.
For example, composite lumber is currently used for decking, railing systems and playground equipment. Sources indicate that there currently exists a $300 million per year market for composite lumber in the United States. It is estimated that 80% of the current market uses a form of wood plastic composite (WPC). It is estimated that the other 30% is solid plastic. A wood plastic composite (WPC) refers to any composite that contains wood particles mixed with a thermaloset or thermoplastic. The WPC industry uses common wood species related to their region for the United States including pine, maple, oak and others. Particle sizes that are typically incorporated into WPC's range from 10 to 80% mesh. The presence of wood fiber increases the internal strength and mechanical properties of the composite as compared to, e.g., wood flower. WPC uses approximately 20% to 70% by mass wood to plastic ratios in a typical manufacturing process.
WPC's have desirable characteristics as compared to plastic systems. For example, the addition of wood fillers into plastic generally improves stiffness, reduces the coefficient of thermal expansion, reduces cost, helps to simulate the feel of real wood, produces a rough texture improving skid resistance, and allows WPC to be cut, shaped and fastened in a manner similar to wood.
The addition of wood particles to plastic also results in some undesirable characteristics. For example, wood particles may rot and are susceptible to fungal attack, wood particles can absorb moisture, wood particles are on the surface of a WPC member can be destroyed by freeze and thaw cycling, wood particles are susceptible to absorbing environmental staining, e.g., from tree leaves, wood particles can create pockets if improperly distributed in a WPC material, which may result in a failure risk that cannot be detected by visual inspection, and wood particles create manufacturing difficulties in maintaining consistent colors because of the variety of wood species color absorption is not consistent. Plastics use UV stabilizers that fade over time. As a result, the wood particles on the surface tend to undergo environmental bleaching. Consequently, repairing a deck is difficult due to color variation after 6 months to a year of sun exposure.
In a typical extrusion composite design, increased load bearing capacity capability may be increased while minimizing weight by incorporating internal support structures with internal foam cores. Examples of such designs are taught in U.S. Pat. Nos. 4,795,666; 5,728,330; 5,972,475; 6,226,944; and 6,233,892.
Increased load bearing capacity, stability and strength of non-extruded composites has been accomplished by locating geometrically shaped core material in between structural layers. Examples of pre-formed geometrically shaped core materials include hexagon sheet material and lightweight woods and foam. Problems associated with typical pre-formed core materials include difficulties associated with incorporating the materials into the extrusion process due to the pre-formed shape of the materials.
Other efforts to increase strength with composite fiber design have focused on fiber orientation in the composite to obtain increased strength to flex ratios. In a typical extrusion composite process, the fiber/fillers are randomly placed throughout the resin/plastic. Therefore increasing strength by fiber orientation is not applicable to an extrusion process.
Foam core material has been used in composites for composite material stiffening, e.g., in the marine industry, since the late 1930's and 1940's and in the aerospace industry since the incorporation of fiber reinforced plastics.
Recently, structural foam for core materials has greatly improved in strength and environmental stability. Structural core material strengths can be significantly improved by adding fibers. Polyurethane foams can be modified with chopped glass fibers to increase flexible yield strength from 8,900 psi-62,700 psi.
Prior art patents tend to describe foam core materials as rigid or having a high-density. However structural mechanical properties of the foam core tend not to be addressed. A common method to obtain a change in load capacity is to change the density of the material. For example, this can be done in a polyurethane in which water is being used as a blowing agent. The density of a polyurethane decreases with the increase in water concentration.
One problem that may occur when a core material and a structural material are not compatible both chemically and physically is delamination. Chemical and physical incompatibility can result in composite structures that suffer structural failures when the core material and the structural material separate from one another.
As discussed above, even though increasing load bearing capacity, stability and strength can be increased by engineering improvements with new resins/plastics, fibers/fillers and internal structural support members, load bearing capacity is still limited by the mechanics of the extrusion process. Despite the advantages associated with engineered building materials, i.e., elimination of problems associated with dry rot, termite, warping, splitting, etc., the failure of extruded composite structural materials to achieve the mechanical attributes of wood has detracted from the potential economic market value of engineered building materials.
Additionally, other applications, such as aerospace applications, utilize composite structures and have had to contend with problems associated with delamination of core materials and structural materials.
Therefore, it is desirable to bring structural core materials to the highest structural load bearing capability possible so that these technologies can be incorporated into extruded composites to replace wood load bearing structures and improve the composite industry as a whole by stabilizing the composite core to help improve composite core materials from delamination.